1. Field of the Invention
The present invention relates to gas turbine engines, and relates more particularly to an improved gas turbine engine and method and control system therefore particularly useful as the power plant for a ground vehicle or advanced rotorcraft.
2. Description of Related Art
A variety of gas turbine devices are known in the art for generating useful work by combusting fuel mixtures. Of such devices, many gas turbine are directed to powering vehicles, such as ground, water and air vehicles.
Gas turbine engine fuel efficiency has improved in recent years to levels typically associated with reciprocating engines. In most cases, gas turbine engines have displaced reciprocating engines as the engine of choice for larger aircraft as a result of their relative light weight, high power output and greater reliability. However, for applications such as the next generation of advanced helicopters and ground vehicles, the ability to efficiently deliver power at lower output shaft speeds remains a challenge. Typical gas turbine engines for these applications are of the free turbine type, that is, the output turbine is driven by the gases produced by a gas generator system. This approach is satisfactory when the output speed of the system is within a narrow speed range, but as the output speed of the engine drops, the power turbine efficiency falls off.
Vehicles, such as advanced unmanned helicopters, are expected to utilize variable speed rotor systems that reduce the speed of the main rotor to improve aerodynamic efficiency. In order to take advantage of this significant improvement in rotor efficiency, the engine must be capable of providing low fuel consumption at an output shaft speed of roughly half of that required for takeoff conditions. Attempting to operate a conventional free turbine engine at output shaft speeds lower than 75% typically results in a loss of power turbine rotor efficiency. What physically occurs is that the turbine loading, in terms of work per unit airflow remains constant, while the speed decreases, robbing the turbine of work capability. This results in overloading of the turbine and reduced turbine efficiency. At the same time, the gas generator turbine experiences a reduced loading since the work being done by the gas generator rotor is reduced as the engine slows down and the inlet airflow and pressure ratio decreases.
Discussions of exemplary prior art gas turbine engine structures may be found in U.S. Pat. Nos. 3,237,404, 3,660,976, 3,899,877 and 3,941,015. These references are generally directed to schemes for transmitting motive power from the gas generator to the engine output shaft. The positive performance benefits of coupling the output turbine to the gas generator have been shown by the prior art (e.g., Woodhouse et all, and Flanigan et all). However, these prior art systems are all based on concepts in which there was a fundamental speed difference between the shafting systems being coupled, and therefore these systems required the addition of gear trains to accommodate these speed differences. However, such systems, in employing intermediate gearing, add significant expense complexity and weight to the system. Thus, there remains a continuing need in the art for improved gas turbine engine systems that address the above concerns. The present invention provides a solution for these problems.